Oligonucleotide compounds have important therapeutic applications in medicine. Oligonucleotides can be used to silence genes that are responsible for a particular disease. Gene-silencing prevents formation of a protein by inhibiting translation. Importantly, gene-silencing agents are a promising alternative to traditional small, organic compounds that inhibit the function of the protein linked to the disease. siRNA, antisense RNA, and micro-RNA are oligonucleotides that prevent the formation of proteins by gene-silencing.
RNA interference or “RNAi” is a term initially coined by Fire and co-workers to describe the observation that double-stranded RNA (dsRNA) can block gene expression (Fire et al. (1998) Nature 391, 806-811; Elbashir et al. (2001) Genes Dev. 15, 188-200). Short dsRNA directs gene-specific, post-transcriptional silencing in many organisms, including vertebrates, and has provided a new tool for studying gene function. RNAi is mediated by RNA-induced silencing complex (RISC), a sequence-specific, multi-component nuclease that destroys messenger RNAs homologous to the silencing trigger. RISC is known to contain short RNAs (approximately 22 nucleotides) derived from the double-stranded RNA trigger, but the protein components of this activity remained unknown.
siRNA compounds are promising agents for a variety of diagnostic and therapeutic purposes. siRNA compounds can be used to identify the function of a gene. In addition, siRNA compounds offer enormous potential as a new type of pharmaceutical agent which acts by silencing disease-causing genes. Research is currently underway to develop interference RNA therapeutic agents for the treatment of many diseases including central-nervous-system diseases, inflammatory diseases, metabolic disorders, oncology, infectious diseases, and ocular disease.
Despite the different synthetic strategies developed for conjugation of various ligands to the oligonucleotides, the synthesis of ligand-oligonucleotide conjugates is anything but trivial and requires extensive expertise in organic chemistry and solid-phase synthesis. A real advance would be to use a coupling reaction that can be utilized for a large variety of ligands and oligonucleotides. The Huisgen 1,3-dipolar cycloaddition of alkynes and azides, the “click” reaction, is especially attractive for irreversible coupling of two molecules under mild conditions. The “click” chemistry has recently emerged as an efficient strategy to conjugate carbohydrates, peptides and proteins, fluorescent labels and lipids to oligonucleotides. Therefore, there is a clear need for new reagents that can be utilize for “click” chemistry for conjugation of ligands to oligonucleotides. The present invention is directed to this very important end.